The present invention relates to an Interrogator and Tags of a wireless communication system in which the Interrogator transmits a carrier signal to the Tags, each of which modulates the received carrier signal and reflects the modulated signal to the Interrogator.
Conventionally, a communicating system configured such that an Interrogator which transmits a main carrier signal to one or more Tags and each of the Tags modulates the main carrier signal with a subcarrier signal and reflects the thus modulated carrier signal to the Interrogator has been known. An Example of such a communication system is disclosed in EP 0 853 392 A2, teachings of which are incorporated herein by reference. In this publication, by employing a frequency dividing multiple access method using a plurality of subcarrier signals, communication with the plurality of Tags is processed.
However, if the number of the Tags are relatively large, it is difficult to use different subcarrier frequencies for respective Tags, and the possibility of occurrence of a so-called “collision ” is relatively large. It may be possible to configure the Tags to change the subcarrier frequency. However, even in such a case, each Tags cannot identify which subcarrier frequencies are used by other Tags, and therefore, the collision may still occur.
Further, the power sources of the Tags are generally small due to their structures, and thus the power supplied thereto is relatively small.
Furthermore, if there are a plurality of Interrogators, the frequency of the main carrier signal may be changed according to the conventional communication system. However, the Tags typically do not have selectivity for frequencies. Therefore, unexpected Tags as well as the intended Tags may reflect the modulated carrier signals, and the possibility of the collision may increase in such a system.